Matt Damon is 27 - the same age as Marlon Brando when he appeared in the film version of "A Streetcar Named Desire.

" But it's impossible to imagine a sweat-soaked Damon, dressed in a ripped T-shirt and screaming, "Hey, Stel-la!

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" Despite his age, Damon is still a boy. He's got the baby face, the "aw, shucks" affect and the Average Joe bod. And he's not alone there are a whole bunch of twenty and thirtysomething movie actors who look and act as mature as the lead characters in "South Park.

" David Schwimmer? He may be 31, but he's most definitely still a boy. Christian Slater? Twenty-nine and a widdle baby after all these years. The same goes for Ethan Hawke (27), Matthew Broderick (a ripe old 36), Jim Carrey (also doddering at 36), Hugh Grant (37), Brad Pitt (33) and a host of others. Time was when film actors were manly men, and there was no question about it. Robert Mitchum was only 30 when he starred in "Out of the Past," and no one ever accused ol' Mitch of an extended boyhood. Same goes for Gregory Peck (29 in "Spellbound"), Warren Beatty (30 in "Bonnie and Clyde"), Steve McQueen (30 in "The Magnificent Seven") and lots of others. Heck - Mel Gibson was 25 when he made "The Road Warrior," and if that isn't a hunk of macho man, what is? But here's what passes for manhood these days: In the recent film "The Newton Boys," about a group of Depression-era bank robbers, the outlaws are portrayed by Skeet Ulrich, Ethan Hawke, Matthew McConaughey and Vincent D'Onofrio. Okay, we'll give you McConaughey, a studly kind of guy. But the rest of this outfit is about as mature and frightening as the Teletubbies. (Ethan Hawke with a gun? Puh-leeze!) It's a new world out there in movie star land. "Hunk" has been redefined to mean boyish, androgynous, sexually nonthreatening. Out with washboard abs and killer pecs! In with sensitive looks and cute, but not killer, bods. Wanna blame someone for this state of affairs? Start with changing attitudes toward sexuality - then move on to what rocks the world of teenage girls these days. "What's more permissible today [in the movies] is the androgynous male," says Foster Hirsch, author of "Acting Hollywood Style.

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" "There's a greater relaxation about sexuality and about sexual categories. It's more permissible to have a kind of slippage between what used to be considered masculinity and androgyny.

" "The teen audience has pretty much taken over the box office," says Michael Musto, the entertainment and nightlife columnist for The Village Voice. "What they demand is ambiguous, youngish, somehow sexually amorphous heartthrobs, as opposed to hunks, because it's less threatening to them.

" Now we're not suggesting that all of today's young movie stars are little more than wussy-boys. Guys like Nicolas Cage, Will Smith, Ben Affleck, Vince Vaughn and Keanu Reeves register strongly on the hunk-o-meter. It's just that there are so many Leonardo DiCaprio types out there, you have to wonder what's going on in the old Zeitgeist. "Part of it is that we don't have any role models for adults anymore," says "Sex and the City" author Candace Bushnell. "Even our President acts like an adolescent. Everyone seems to be stuck in this perpetual state of adolescence. You have this whole group of people for whom `grownup' means their parents; even baby boomers [feel that way]. So there's no model, someone who has responsibilities and behaves in an adult way.

" It's almost as if we've become a nation of people who don't want to grow up. And we've decided that we want our movie stars to reflect the same desire. "It's the Peter Pan syndrome extended to the whole culture," says Bushnell. But there's also a sense in which the current vogue for baby-faced leads is a response to certain excesses of the past. "In some ways, it's a reaction to the biggest stars of the Reagan era, who were musclebound men [like] Schwarzenegger and Stallone," says Hirsch. "Their primary appeal was to adolescent males. The primary appeal now is to adolescent girls.

" Yes, a lot of the blame for this state of affairs can be laid squarely at the feet of all those lusting teenyboppers who went to see "Titanic" time after time because Leonardo is so cute! But it's not just young females who have made the likes of Ethan Hawke into movie stars. Stats-wise, 12-to-20-year-olds account for only 15% of the U.

S. population older than 12, but they purchase slightly more than one-quarter of all movie tickets sold to those 12 and older. And a recent Motion Picture Association of America survey found that almost half of 12-to-17-year-olds see a movie at least once a month, compared with only 26% of people 18 and older. But it's teen girls in particular, who tend to go to movies in groups, who have been the primary movers and shakers behind such hits as "Clueless," "William Shakespeare's `Romeo and Juliet' " (featuring that hot teen love duo of DiCaprio and Claire Danes), "Scream," "Scream 2" and "I Know What You Did Last Summer.

" Think movie marketing types aren't aware of this phenomenon? Robert Levin, head of worldwide marketing at Sony Pictures Entertainment, has said: "A while ago everyone was focusing on the young male audience: young male action movies, young male comedies. Now we're saying `Wow, there's really a young female audience out there that we must be attentive to.

' " So as long as these teens continue to have box-office clout and U.

S. Census projections say the number of under-20s will expand through at least 2010 the longer pretty boys like Skeet Ulrich, Jason Priestley and Brendan Fraser will dominate movie screens. But will these teen dreams be trapped in an eternal onscreen adolescence? Or will they be allowed to age along with their audience? Only time will tell. "The [actors] who are able to mature into the next level are going to have a long career," Musto says. "Leonardo has the capability because he's a real actor. But the ones who will be stuck playing boys at 40 it's not a pretty sight.

" Then, referring to an actor who seemed stuck in juvenile roles his entire career, Musto adds, "Even Michael Crawford became `The Phantom of the Opera.